1. Field of the Invention
The present invention relates to a running device for a track-laying vehicle in which a falling height of a nose is reduced while a local impact is relaxed when a crawler crosses over a projected running surface.
2. Description of Related Art
Conventionally, sometimes a bogie type (oscillation type) lower track roller, particularly a double-bogie type lower track roller is mounted on the widely used running device for the track-laying vehicle in order to improve ride quality during running. FIG. 9 illustrates a bulldozer that is of an example of the track-laying vehicle. The track-laying vehicle includes a pair of running devices in right and left lower portions of a vehicle body, and the running device includes a crawler 6. A track frame 1, an idler 2, a sprocket 3, and double-bogie devices 10 are provided in the running device. The track frame 1 is provided along a front-back direction of the vehicle. The idler 2 and the sprocket 3 are provided in front-back end portions, respectively. The double-bogie device 10 includes a plurality of lower track rollers 11 and 12 rotatably supported in a lower portion of the track frame 1 and supported in the front-back direction of the vehicle body while freely oscillated.
For example, in a running device of a track-laying vehicle disclosed in Japanese Patent Application Laid-Open No. 2001-225770, a predetermined tension is applied outward in a yoke while a spring is interposed therebetween, and the yoke is mounted on a front end portion of a track frame while being movable in a longitudinal direction (front-back direction of vehicle). An idler is rotatably attached to a leading end portion of the yoke. A sprocket is attached to a vehicle body provided close to a rear and of the track frame, and a bogie type lower track roller unit is provided between the idler and the sprocket while being able to be oscillated. In the running device disclosed in JP-A No. 2001-225770, the bogie device provided close to the idler is a single-bogie device that supports one lower track roller while being freely oscillated. A plurality of sets of double-bogie devices are provided in the track frame between the sprocket and the single-bogie device. A crawler is entrained about the idler, the sprocket, and the plurality of lower track rollers.
For example, as illustrated in FIG. 1, JP-A No. 2005-225485 discloses a running device for track-laying vehicle including a single-bogie device 40a. In the single-bogie device 40a, the center of a leading-end-portion arm 49 is turnably supported by a pin 42 attached to a front end portion of the track frame 1, the idler 2 is rotatably supported in a front end portion of the leading-end-portion arm 49, and a leading-end-portion lower track roller 43 is rotatably supported in a rear end portion of the leading-end-portion arm 49. A so-called single-bogie device 40b is provided in a rear end portion of the track frame 1. In the single-bogie device 40b, a front end portion of a rear-end-portion arm 45 is attached while being able to be oscillated, and a rear-end-portion lower track roller 47 is rotatably attached to a rear end portion of the rear-end-portion arm 45. The plurality of sets of double-bogie devices 10 are provided in the lower portion of the track frame 1 between the front single-bogie device and the back single-bogie device. The entire structures of the running device and double-bogie device are visually similar to those of an embodiment of the invention. Therefore, for the sake of convenience, the numerals in the corresponding portions of the conventional running device and double-bogie device are designated in principle by the same numerals as the embodiment of the invention.
As illustrated by a solid line and a phantom line (alternate long and two short dashes line) of FIG. 2, in the conventional double-bogie device 10, while freely oscillated, a base end portion of a first arm 31 is supported in the lower portion of the track frame 1 with a first pin 33 as a first turning fulcrum. While freely oscillated, a substantially central portion of a second arm 32 is supported in the leading end portion of the first arm 31 with a second pin 35 as a second turning fulcrum. First and second lower track rollers 11 and 12 are rotatably attached to front-back end portions of the second arm 32, respectively. At this point, the first pin 33 is provided in a position closer to the idler 2 than the second pin 35. Elastic members 34a and 34b are fixed in an abutment portion between a leading-end upper portion of the first arm 31 and the lower portion of the track frame 1. The abutment of the elastic members 34a and 34b restricts the upward turning of the first arm 31 while absorbing impacts of the first and second lower track roller 11 and 12 when the running device runs on or runs over the raised portion. An apparent spring constant of the bogie device is derived from the elastic members 34a and 34b. 
When the crawler 6 runs on a flat ground, the first and second lower track rollers 11 and 12 are located in a horizontal position, and the both the lower track rollers 11 and 12 come equally into contact with a wheel tread of a connecting link 62 connected to the crawler 6. Therefore, a position of a load applied onto the pair of front-back first and second lower track rollers 11 and 12 of the double-bogie device 10 becomes the center of the first arm 31, which is of the central support portion between the first and second lower track rollers 11 and 12. The bogie device 10 is subjected to the vehicle body load in the first-arm central position (second turning fulcrum). On the other hand, when the crawler 6 runs on the raised portion, the pair of front-back first and second lower track rollers 11 and 12 of the double-bogie device 10 is turned clockwise or counterclockwise by the inclination of the crawler 6 while the second arm 32 is interposed therebetween. Stopper portions 32b and 32c raised from upper surfaces of front-back arm portions which sandwich the turning fulcrum (second turning fulcrum) of the second arm 32 therebetween abut on a lower surface of the first arm 31, and the abutment surface is subjected to the vehicle body load. At this point, the impact is relaxed by elastic forces of the elastic members 34a and 34b that are fixed to portions opposite the lower surface of the track frame 1 and the upper surface of the first arm 31.
Thus, a position of the load applied between the pair of front-back first and second lower track rollers 11 and 12 of the double-bogie device 10 varies according to the shape of the actual contact area. For example, the position of the load applied between the pair of front-back first and second lower track rollers 11 and 12 becomes the central fulcrum between the first and second lower track rollers 11 and 12 when the crawler 6 runs on the flat surface, and the position becomes one of the front-back stopper abutment surfaces of the second arm 32 when the crawler 6 runs on the raised portion. At this point, as schematically illustrated in FIGS. 2, 10A, and 10B, the apparent spring constant of the double-bogie device 10 depends on a distance L2 from the oscillation fulcrum of the first arm 31 to the central fulcrum (second turning fulcrum) between the first and second lower track rollers 11 and 12 and distances L1 and L3 (L1<L3) from the oscillation fulcrum of the first arm 31 to abutment surfaces between the oscillation fulcrum of the first arm 31 and front-back stopper portions 32b and 32c of the second arm 32. For example, when the abutment surface of the front-portion stopper portion 32b of the second arm 32 closest to the oscillation fulcrum of the first arm 31 is subjected to the vehicle body load, a distance between the first turning fulcrum and the abutment portion of the elastic bodies 34a and 34b, which is of an action point, is kept constant to minimize a distance between the power point and the first turning fulcrum. Therefore, the spring constant is maximized. FIG. 10B illustrates a distribution of the apparent spring constant in position (power point) subjected to each load. That is, in the distribution of the apparent spring constant of the double-bogie device 10, the apparent spring constant is gradually decreased in the order of the front-portion stopper abutting position of the second arm 32 on the oscillation fulcrum side (idler side) of the first arm 31 of the double-bogie device 10, the oscillation fulcrum of the second arm 32, and the rear-portion stopper abutting position of the second arm on the sprocket side farthest from the oscillation fulcrum of the first arm 31. In FIGS. 10B and 10C, a horizontal axis corresponds to the crawler of FIG. 10A in terms of position.
In the running device of the conventional track-laying vehicle, when the track-laying vehicle runs on the raised portion, the position supporting the vehicle body load is a position in which the elastic members 44a and 44b are placed in the single-bogie device 40a or 40b, and the position supporting the vehicle body load is substantially matched with the position of the leading-end-portion lower track roller 43 like the flat contact area. On the other hand, in the double-bogie device 10, part of the crawler 6 becomes a reverse camber state so as to wrap the raised contact area in the front-back direction. As illustrated with arrows in FIG. 11, in the second arms 32 and 32 of two sets of double-bogie devices 10 and 10 that are adjacent in the front-back direction with the raised contact area sandwiched therebetween, a front side (left of FIG. 11) in the running direction is rotated counterclockwise about a first arm support point, and a rear side (right of FIG. 11) in the running direction is rotated clockwise about the first arm support point. When the oscillation angle (rotation angle of second arm) α reaches a predetermined angle, an abutment surface of the stopper portion 32b, which is of an upper raised surface formed in the arm portion upper surface of the second lower track roller (right of FIG. 11) 12 of the second arm 32 on the front side in the running direction, comes into contact with the lower surface of the first arm 31 to control the further oscillation. At this point, it is assumed that a maximum oscillation angle is an absolute value of the oscillation angle α from a horizontal line.
On the other hand, as illustrated in FIG. 11, in the running device of the track-laying vehicle, the crawler 6 includes a shoe plate 61 and a connecting link 62 that is fixed to an inner surface (non-contact area) of the central portion in a width direction of the shoe plate 61 by a bolt (not illustrated). In FIG. 11, a grouser portion 61a is projected at a contact area-side rear end of the shoe plate 61, and tongue-shaped front-back wrap portions 61b and 61c are extended in the front-back direction at front-back ends of the shoe plate 61. The connecting links 62 are connected to each other by press-fitting connecting pins 63 and crawler bushes 64 in connecting holes made in front-back end portions of the connecting link 62. The connecting links 62 fixed to the shoe plates 61 are connected in an endless manner to form the crawler 6. The connected connecting links 62 can be turned about the connecting pin.
The front-back wrap portions 61b and 61c of the shoe plate 61, which are connected with the connecting link 62 interposed therebetween, overlap each other during the connection. In FIG. 11, the front wrap portion 61b is extended toward the contact area while inclined downward, and a surface opposite the contact area side is formed as a convexly curved surface. The back wrap portion 61c is extended toward the surface opposite the contact area side while inclined upward, and a surface on the contact area side is formed as a concavely curved surface. At this point, in the shoe plates 61 connected in the front-back direction, a corner portion formed between the front wrap portion 61b of the shoe plate 61 and the base end of the grouser portion 61a and a leading end of the back wrap portion 61c are relatively turned while slid on each other, and the crawler 6 becomes the reverse camber state so as to wrap the contact area when running over the raised contact area. The reverse camber angle is restricted by a crossing angle β of the connecting link 62. The crossing angle β is determined by a sliding portion shape such as a wrap angle formed between the front wrap portion 61b and the back wrap portion 61c. In other words, the crossing angle β means a maximum reverse camber angle, and usually the crossing angle β is determined in designing the crawler. In the conventional crawler, the maximum angle of the oscillation angle α of the second arm 32 is set substantially equal to the crossing angle β.
As described above, in this kind of running device including the running device for track-laying vehicle disclosed in JP-A Nos. 2001-225770 and 2005-225485, the maximum angle of the oscillation angle α of the second arm 32 is designed substantially equal to the maximum angle of the crossing angle β between the adjacent connecting links. For example, in a certain vehicle, the crossing angle β is set to 7°30 ′ with respect to the connecting link that is of a reference, and the maximum angle of the oscillation angle α ranges with in ±8° with respect to the horizontal line. When each of the reverse camber angle and the oscillation angle α reaches the maximum angle, the crawler is not further bent, and the second arm cannot further be turned (oscillated). That is, during the running, when part of the crawler runs on the raised contact area to reach the maximum reverse camber state, and when one of the first lower track roller and the second lower track roller of the double-bogie device exists in the corresponding position, the oscillation of the second arm also reaches the maximum. When the raised contact area exists below the first lower track roller closer to the idler, the front-portion stopper of the second arm on the first-lower-track-roller supporting side comes into contact with the lower surface of the first arm, an operator is subjected to the impact stronger than that of other regions due to a high apparent spring constant. Additionally, at this point, the crawler goes straight, and the crawler is orientated obliquely upward by the track frame while the reverse camber portion on top of the raised contact area is sandwiched as a whole. Once the crawler runs over the raised surface to move the gravity of the vehicle to the front side of the raised surface, the vehicle is largely turned about the raised contact area, and the leading end of the crawler falls rapidly. Possibly the impact applied in the falling and the falling height provide an uncomfortable feeling to the operator.